1. Field of the Invention
The present invention relates to methods for imprinting micro-patterns, and relates in particular to a technique of evanescent (proximity)-field-assisted fabrication of two-dimensional micro-patterns of smaller dimensions than the wavelength of light.
2. Description of the Related Art
A typical example of fabrication techniques for making micro-circuit patterns on a semiconductor substrate is photolithography. In this technique, a photo-sensitive material (photo-resist) is coated on a substrate base, and a reduced image of a masking pattern containing micro-circuit patterns is projected on the base by optical means to expose the photo-resist material. The degree of resolution achievable by photolithography is limited by light diffraction effects, and the minimum line width is ultimately limited by the wavelength with respect to light. For this reason, it is necessary to use shorter wavelengths for fabrication of finer patterns. Currently, this is being achieved by using the g-line (436 nm wavelength) or the i-line (365 nm wavelength) of a mercury lamp, or a KrF excimer laser (248 nm wavelength) or a ArF excimer laser (193 nm wavelength), and the trend is towards the use of shorter wavelengths to meet the demand for finer line width.
In recent years, active research has been conducted on the use of evanescent fields (proximity fields) for fabrication of micro-patterns. An evanescent field is an electro-magnetic field produced when light is transmitted through a transmissive object placed at a sub-wavelength distance from a light source. If an object having a micro-pattern surface structure, comprised by high and low structures, is placed within a sub-wavelength distance of a light source, light is transmitted through high structures, and evanescent fields are generated at these locations. The evanescent field diminishes exponentially as the separation distance increases beyond the wavelength of transmitting light. Therefore, if a micro-pattern with a height difference on the order of several tens of nanometers is coated with a photo-sensitive material and is placed at a sub-wavelength distance to an exposure light source, light is transmitted only through the high structures, thereby exposing only the coating on the high structures of the micro-pattern to the exposure light. In this technique, the line width of the exposed material is governed only by the fineness of the pattern placed in proximity of the light source, not by the exposure wavelength. Therefore, it is possible to produce micro-patterns exceeding the limit imposed by the exposure wavelength.
A known example of micro-pattern fabrication based on the evanescent field effect uses an optical fiber having one end sharpened to a sub-wavelength dimension, and a laser light is injected from the opposite end of the fiber. The sharpened end is placed in contact with or in proximity to (at a sub-wavelength distance) a surface of a substrate base coated with a photo-resist film, then a proximity field is produced in the vicinity of the exposed region, and the light is transmitted through the proximity field and the photo-resist film is exposed to the light. Therefore, by sharpening the tip to a sub-wavelength dimension, it is possible to expose a pattern scribed by a line width of a sub-wavelength dimension on the photo-resist film. The photo-resist film is developed by photo-lithographical technology, then, using the exposed sections of the photo-resist film as etch-masking, a patterned surface of the substrate base is etched to remove the unprotected regions, thereby leaving behind micro-patterns of a sub-wavelength line width.
However, in this technique, the proximity field can only be produced at the tip of the optical fiber opposite to the base, therefore, the exposed pattern is a point. To apply this technique to the production of a two-dimensional pattern of some integrated circuit device, it is necessary to scan the tip in a pattern of the circuit, so that it not only consumes a vast amount pattern-making time but also leads to the need for a complex tip-driving apparatus. Therefore, this technique is considered impractical.
For these reasons, there have been attempts to produce micro-patterns using a mask that has a proximity field exposure pattern and transferring the two-dimensional pattern to a substrate base. For example, a prism made of a light transmissive material such as glass and the like is prepared and a photo-mask, having a proximity field exposure pattern of sub-wavelength dimensions, is attached to the bottom surface of the prism. Light is injected into the prism at such an angle that it is totally reflected at the bottom surface of the prism. Next, a substrate base coated with a photo-resist film is placed at a sub-wavelength distance of a proximity exposure pattern so that an evanescent field is produced and a two-dimensional pattern, conforming to the proximity exposure pattern, is exposed on the photo-resist film. In this process, an optical system for injecting a laser beam from an inclined surface of a prism is used, and the incident laser beam is totally reflected at a plane having a proximity field exposure pattern; and then it is transmitted to the outside through another inclined surface. A photo-resist film surface of a substrate base is made to contact closely with the proximity field exposure pattern so as to generate an evanescent field to propagate exposure light along the exposure pattern, and thereby producing micro-patterns having line widths of sub-wavelength dimensions.
The method utilizing the evanescent field described above enables one to produce a two-dimensional micro-pattern having line widths less than the wavelength of the exposure light on a photo-resist film of a substrate. However, the method requires that the incident beam be aligned with the inclination angle of a prism, and the optical system is necessarily complex. Also, the exposure pattern section can only accept a small exposure area. And, because the incident light is at an angle to the proximity field exposure pattern, the depth of imprinting is shallow, and because the exposed area increases quickly along the beam line, it is difficult to expose a structure having high aspect ratios on the photo-resist film.